The magnets are pretty strong. I have seen a couple of instances of people using these pumps on 3/4" tanks without problem. I think I even saw one of it being used on 1" glass, but I'm not sure that may have just been a question asking if it would work.
I'm not sure what you mean by controlling the flow.
You can control the frequency of the wave by the knob on the controller.
You can control the speed of the pump with an LED dimmer or PWN dimmer reducing the power going into the controller or you can control the speed of the pump by using an Apex or Reef Angel to reduce the power going to the pump.
The only way to control or adjust the flow would be to modify the housing in some way and no one has really experimented with that except Ron Reefman on the WP-25 thread, but I don't know what the outcome of mods were.